The present invention relates to an image control apparatus for a television receiver, and more particularly to an image control apparatus of this kind which is adapted to remove distorted waveforms, such as ghosts, and noises to improve the image quality.
One of problems inherent to television signals emitted from a broadcasting station is the distorted waveform, such as a ghost. A technique for removing such distorted waveform is disclosed, for example, in JP-1-59791.
FIG. 1 is a block diagram illustrating an example of a prior art image control apparatus for a television receiver. In FIG. 1, reference numeral 101 designates a waveform equalizing means, 201 a storage means, 202 a distorted waveform detecting unit, 203 a processing unit, 204 a waveform equalizing means control unit, 210 a noise detecting unit, and 104 an image quality control unit.
The waveform equalizing means 101 removes distorted waveform from a video signal fed from a video signal detecting circuit. The storage means 201 synchronously adds the video signal to reduce noises included therein, as well as stores the video signal thus noise reduced. The distorted waveform detecting unit 202 exclusively extracts a GCR signal (Ghost Cancel Reference Signal: refer to Official Gazette No. 155, Aug. 1, 1989, Order of Post Service Ministry No. 48 for detail) or a reference signal such as a synchronizing signal from an input or output signal of the waveform equalizing means 101, and compares this reference signal with a base signal stored in the distorted waveform detecting unit 202 to detect distorted waveform from the difference therebetween. Further, the processing unit 203 generates data corresponding to each frequency for controlling the waveform equalizing means 101 from an output from the distorted waveform detecting unit 202 such that the waveform equalizing means 101 has the characteristic for removing the detected distortion in waveform. Thus, the waveform equalizing means control unit 204 controls the waveform equalizing means 101 in accordance with the output of the processing unit 203 to perform suitable waveform equalization. The noise detecting unit 210 examines the magnitude of noise from data stored in the storage means 201. If the magnitude is above a predetermined value, the noise detecting unit 210 controls the image quality control unit 104 to degrade the image quality so as to make the noise invisible.
The above-mentioned prior art utilizes a waveform equalizing means for removing distorted waveform so as to make the characteristic of a video signal uniform from a low frequency band to a high frequency band, and only when the signal-to-noise (S/N) ratio presents a low value, the image quality is automatically degraded to provide an acceptably clear image. Stated another way, with a low S/N ratio, an acceptable image may be provided by degrading the image quality even if the waveform equalization is not perfectly performed. Therefore, it can be said that this method is intended to merely compensate for imperfect waveform equalization.
In the above-mentioned prior art, even if the S/N ratio of an input signal is not so low, when the input signal has few components in the high frequency band, performing waveform equalization for this signal may result in amplifying not only the high frequency components but also noises, thereby causing a coarse image. However, the image quality control unit cannot produce any effect on such a degraded image.
Further, as a general problem of this kind of waveform equalizing means, when a signal is input for intentionally producing a coarse-effect image, wherein a high frequency band of the frequency characteristic of the signal is excessively raised, the input signal is equalized, in the prior art, to lower the high frequency band thereof to make the overall frequency characteristic flat, which, however, may result in an excessively soft image quality from the viewpoint of visual sense, compared with the image before equalization, thereby giving an impression of a dim image, in comparison with the image produced by the input signal without being equalized.